This application is based upon and claims benefit of priority of Japanese Patent Application No. 2001-181944 filed on Jun. 15, 2001, the content of which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to a device for detecting a tensile force of a seat belt used in an automobile vehicle.
2. Description of Related Art
Recently, a regulation for prohibiting an air-bag operation when a small child is seated on a passenger seat is being discussed. It is necessary to detect a weight and a size of a passenger to meet this regulation. A system for detecting a weight of a passenger is known hitherto. However, a body size of a passenger is difficult to be detected only by the weight detector because a load imposed on a passenger seat increases when a child seat is fixed by a seat belt. To cope with this problem, JP-A-2000-296757 proposes a system which detects a passenger""s weight and size by combining a weight sensor and a tensile force detector of a seat belt.
However, it is very difficult to make a tensile force detector that precisely detects a tensile force at a low level and is durable against a high impact force generated upon an accidental collision or the like. It is required for the seat belt tensile force detector to accurately detect a low level force of about one N (newton) in detecting a passenger size. On the other hand, it has to endure a very high level force in excess of 22,300 N (according to a JIS-D4609) which is imposed upon a vehicle collision. It is a difficult task for the seat belt tensile force detector to satisfy both requirements.
The present invention has been made in view of the above-mentioned problem, and an object of the present invention is to provide an improved seat belt tensile force detector that is able to precisely detect a low level tensile force and is durable against a very high impact force.
A device for detecting a tensile force applied to a seat belt is composed of a tensile force detector having a strain gauge and a protecting plate overlapped on the tensile force detector. The detecting device is installed in the seat belt in line with the tensile force applied to the seat belt. The tensile force detector is composed of an elastic plate that elastically deforms according to the tensile force applied thereto and a strain gauge mounted thereon. While the elastic plate elastically deforms according to the tensile force, it breaks off when a large tensile force exceeding a predetermined level is applied thereto. On the other hand, the protecting plate has a high tensile strength that is able to endure a very large tensile force generated by a vehicle collision or the like.
The detecting device is installed in the seat belt, so that the tensile force is applied only to the elastic plate under a normal situation and is applied to the protecting plate when the elastic plate is broken by a high tensile force. A relatively small tensile force applied to the seat belt by a passenger is precisely measured by the strain gauge mounted on the elastic plate, and a size of a passenger can be detected based on the measured tensile force. On the other hand, when the elastic plate is broken, the seat belt function to protect a passenger is maintained by the protecting plate having a-high tensile strength.
Both of the elastic plate and the protecting plate are flexibly connected to the seat belt via a pair of circular belts so that the tensile force of the seat belt is measured without being influenced by postures of a seat belt buckle or other connecting portions. Both of the elastic plate and the protecting plate have a pair of rectangular slits through which the circular belts are flexible inserted. A distance between two slits of the elastic plate is made smaller than that of the protecting plate, so that the tensile force is applied only to the elastic plate under a normal situation.
Other objects and features of the present invention will become more readily apparent from a better understanding of the preferred embodiment described below with reference to the following drawings.